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by Andrew W. Choi BA, and Steve Xu MD, MSc
Cellulitis refers to a bacterial infection of the deep layers of the skin as well as the tissue below the skin. In the United States alone, there are about 14.5 million cases of cellulitis, which cost $3.7 billion yearly in annual health care costs. Ten percent of the hospitalizations in the United States related to an infection were due to cellulitis.
What Does Cellulitis Look Like?
It usually presents initially as red, swollen skin that feels warm and is tender. This infection can occur anywhere on the skin, but it most commonly happens on the lower legs in adults and on the face or neck in children. If cellulitis is caught early and treated, it usually clears completely without any long-term issues. When it is seen early, oral antibiotics can be given, improving the cellulitis within 24-48 hours. However, without treatment, the infection can spread quickly, even into the bloodstream. If the infection becomes severe enough, hospitalization and IV antibiotics may be required.
The Diagnostic Challenge of Cellulitis
There are many other skin conditions that can look like cellulitis, making it difficult to distinguish from benign conditions, such as a fungal infection or redness from poor circulation. Some common things physicians look for to diagnose cellulitis include red streaking around the area, which would mean the lymph vessels are inflamed, a condition more likely to be seen in cellulitis. Also, physicians may look for a wound or cut where bacteria could have entered the skin. Usually, the patient feels sick and may have fevers and chills. In addition, cellulitis typically only affects one area. Bilateral lower leg cellulitis has been reported, but redness on both legs usually suggests a different condition.
But even these signs are not perfect indicators. Benign skin rashes can look very similar to cellulitis, so it can be difficult sometimes to differentiate cellulitis accurately from other conditions. Below are some common red rashes that can be confused with cellulitis. These benign red rashes tend to have certain characteristics that can be helpful in differentiating them from cellulitis.
Venous stasis dermatitis, also known as gravitational dermatitis, is a very common condition that can cause redness in the legs. This rash usually happens in people who have poor circulation in their legs. The leg veins have one-way valves that help push blood up from the leg back to the heart. As we get older, these valves can weaken and stop working properly, allowing blood to leak out and pool in the legs, causing swelling and redness. Unlike cellulitis, which usually affects one area, venous stasis dermatitis affects both legs. Also, it does not cause patients to feel sick or have a fever since it is benign. Patients typically complain about itchiness rather than pain.
Another skin condition that can occur on the leg and look like cellulitis is gout. Gout happens when crystals form in a joint, usually the big toe, which causes inflammation that leads to redness near the joint. The area is tender, swollen, and warm, like cellulitis. However, these symptoms are not caused by infection. The redness from gout is usually much more localized around the joint where the crystals are located, and the skin around the redness is clearly normal, as opposed to in cellulitis, where the skin around the redness is hard.
Erythema nodosum, another benign cause of redness in the legs, can be confused with cellulitis. In erythema nodosum, there is inflammation of the fat under the skin, which causes tender knots under the skin that are bright red at first. Unlike cellulitis that usually affects one leg, erythema nodosum affects both legs, usually around the shin. It can be triggered by an infection somewhere else, so the patient may have fevers or feel sick, like in cellulitis.
Similar to erythema nodosum, lipodermatosclerosis is another benign skin condition that causes inflammation of fat under the skin. Lipodermatosclerosis causes a painful red rash on the legs, usually as a result of chronic venous insufficiency, which also causes venous stasis dermatitis. It most commonly happens on the legs, like cellulitis. Often, it can involve both legs, like venous stasis dermatitis and unlike cellulitis. Unlike cellulitis, patients usually do not have fevers or feel sick.
Another infection of the skin, tinea pedis, also known as athlete's foot, can cause redness like cellulitis, especially in patients with suppressed immune systems. Tinea pedis is caused by a superficial infection of the skin by fungi that likes humidity, so it typically occurs in patients who use communal pools or showers. Like cellulitis, tinea pedis usually affects only one foot and can be bright red. Unlike cellulitis, the rash is usually itchy and has scales, and is typically in between the toes. Also, tinea pedis does not cause fevers and the patient does not feel sick, unless cellulitis develops because the tinea pedis has allowed bacteria to enter the skin.
Contact dermatitis can also look very similar to cellulitis. Contact dermatitis happens when out skin touches something that can cause a rash. It happens in everyone at least once in our lives. This includes allergic contact dermatitis, which can be caused by touching poison ivy or nickel (in jewelry or belt buckles), and irritant contact dermatitis, which is caused when something irritates our skin enough, such as certain soaps. The rash from contact dermatitis is red, like cellulitis, and can happen within hours to days of the exposure. The rash can be tender and stinging. Unlike cellulitis, the rash does not cause fevers or make the patient sick. Also, the rash from contact dermatitis is usually very distinct from the rest of the skin around it because it only develops where the skin touched the substance; in cellulitis, there is less of a clear distinction between the infected skin and the rest of the skin.
Right now, there isn't a single laboratory test or imaging modality that can confidently diagnose cellulitis. Typically, a dermatologist must put together the patient's physical appearance, blood work, and medical history to come to a diagnosis.
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